On Fri, 6 Dec 2002, Tyler Durden wrote:
> I was wondering if there was any way around the intra-zone congestion issue,
> and whether a user (and possibly the wireless network that extends off of
> the user) could be moved to another LAN, once the first LAN was experiencing
> congestion problems.
Yes, currently you need an AP for each channel you want to use. The user
simply selects another channel to use. It'd be nice if somebody make a AP
that could handle multiple-channels at once at a reasonable price. The
reality is that considering the range of 802.11 devices the potential for
serious flooding is not something you're going to get into except in
commercial venues. A single AP handles several devices on a single channel
(what you would expect in a private residence or from people driving down
your street) with some aplomb.
Do the math: "How many users can you get in a circle that is 600ft
across?"
The biggest problem is except for a small number of channels (out of all
potential channels) they overlap so there isn't a clear break from one
channel to another with regard to congestion.
> (And if not, I could not see a way to have a large,
> extended WiFi cloud without all users in the extended cloud basically
> competing for a single pool of shared bandwdith.)
They are not using the same channel at the same time so it isn't the
problem you imagine. In general if there are several AP's on a street each
will use a different channel. Each channel can handle a fair number of
users without too bad a collision hit.
> As it turns out, 3 (or possibly 4) of WiFi channels/bands (whatever you call
> them in this space) can be used simultaneously without interference. This
> means then that (aside from using Jim Choate's Pringle's can) you can
I didn't invent the Pringles Can antenna so please don't attribute it to
me.
> basically have an extended series of noninterfering (though connected)
> clouds without worry of interference (so long as you judiciously cycle
> wavelengths as you move from region to region).
It's a question of range really. Wi-Fi is very short range, 300-400ft on
average in good conditions. So even if you have an AP in every house on a
block only a handfull will be on any one channel within range of another
(or they all leave them on the default, which isn't uncommon either).
The -real- problem is getting enough of these to use the same channel so
you -can- bridge from one channel to another. Consider the situation where
you have a node on channel 1. And another node on channel 2. The only way
to bridge is to have somebody who has two AP's, one on each of these
channels. So to bridge two nodes on two different channels take two more
nodes doing packet forwarding.
A real world network requires that there be reasonably high bandwidth
feeds into the cloud from the 'regular' Internet. My hope is to ameliorate
this somewhat by placing a secondary layer (eg 900MHz) with longer range
and (perhaps) lower bandwidth.
> Now I guess the question is, as users come on line, does the lack of a
> centralized "wavelength authority" mean significantly decreased performance,
> or is there some kind of self-regulation that will occur as, perhaps, users
> try one wavelength, find out its crowded, and then try another? (Or does
> some WiFi hardware automatically do this? Or, can it operate at 3 or 4
> channels simultaneously, using multilink PPP or something equivalent?)
It is strictly ad hoc, as it should be. The entire point of the cloud is
for it to be distributed end-to-end. It is currently single channel at a
time. No consumer grade equipment I'm aware of (and I don't follow it that
closely day to day) uses multiple channels at one time.
Since any single channel can handle quite a few real connections it isn't
usually a problem.
Buy yourself a AP and a card for your laptop and experiment.
--
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